This invention relates to a method for preventing the formation of deposits in the fuel reformer associated with a fuel cell system when liquid hydrocarbons are employed as a source of hydrogen.
Fuel cells offer advantages over conventional internal combustion engines in certain applications. Fuel cells are usually more efficient and emit less pollutants into the environment when compared to heat engines. Accordingly, fuel cell systems are being proposed for both stationary and mobile applications which have traditionally been occupied by internal combustion engines. Several different types of fuel cells currently exist or are under development. Most types require hydrogen as a fuel which through an electrochemical conversion is used to generate electricity. The resulting electrical charge provides a source of electricity which may be used to power an electric motor. Unfortunately, hydrogen has a number of significant disadvantages as a practical fuel for commercial applications. In addition to being explosive, pure hydrogen is difficult and expensive to store. Therefore, other fuels such as natural gas and methanol which are easily converted to hydrogen at the site of the fuel cell have been proposed, but these fuels also have serious drawbacks. For example, methanol is expensive as a fuel, lacks an extensive distribution network, and presents groundwater pollution problems. Natural gas, while useful for stationary applications, is less practical for widespread use as a transportation fuel due to its storage and handling problems. Jet, diesel, gasoline and various refinery-blending streams have been suggested as a suitable alternative fuel for use in fuel cells if the sulfur content is reduced sufficiently. See U.S. Pat. No. 6,156,084. Gasoline has the advantage over other fuels of being relatively inexpensive, of already being widely available through a commercial distribution network, and of lacking the storage problems associated with gases such as hydrogen and natural gas.
Before a liquid hydrocarbon, such as gasoline, can be used to fuel a fuel cell, it must first be converted to hydrogen. This processing step is typically carried out in a fuel reformer which is included as an integral part of the fuel cell system. In the fuel reformer, the liquid hydrocarbon is usually vaporized in a heated chamber and passed over an active catalyst which converts the hydrocarbon into hydrogen and carbon dioxide. Typically the liquid hydrocarbon is sprayed into the heated vaporization chamber of the fuel reformer under pressure through one or more orifices opening into the chamber. It has not been previously recognized that deposits will form in these orifices. These deposits will eventually lead to the plugging of the orifice and inoperability of the fuel reformer. This problem has not been observed when gaseous hydrocarbons, such as methane or propane, are used as the fuel but appears to be unique to liquid hydrocarbon fuels. This problem becomes particularly acute when the liquid hydrocarbon is sprayed intermittently as would be expected in a fuel cell system used in a vehicle. However, even in fuel reformers which operate steadily as found in stationary fuel cell systems, deposits may build up over time eventually resulting in a loss of efficiency or in inoperability. Therefore, in order to insure long-term operation in a commercially viable fuel cell system, some method for controlling the formation of deposits is essential when a liquid hydrocarbon is used as the hydrogen source.
The use of detergents and other additive packages have been described for use in fuels intended for internal combustion engines. See for example U.S. Pat. Nos. 5,749,929 and 6,117,197. However, such additives previously have not been described as necessary for use with fuels intended for fuel cells.
As used in this disclosure the word xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase xe2x80x9cconsists essentially ofxe2x80x9d or xe2x80x9cconsisting essentially ofxe2x80x9d is intended to mean the exclusion of other elements of any essential significance to the composition. The phrases xe2x80x9cconsisting ofxe2x80x9d or xe2x80x9cconsists ofxe2x80x9d are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
The present invention is directed to a method for controlling the deposits in the fuel vaporizer of a fuel reformer used to prepare a liquid hydrocarbon for use as a fuel in a fuel cell which comprises introducing into the fuel vaporizer a liquid hydrocarbon containing an effective deposit controlling amount of a nitrogen-containing detergent additive. Useful nitrogen-containing detergent additives according to the present invention include aliphatic hydrocarbyl amines, hydrocarbyl poly(oxyalkylene) amines, hydrocarbyl-substituted succinimides, Mannich reaction products, amino aromatic esters of polyalkylphenoxyalkanols, polyalkylphenoxyaminoalkanes, and mixtures thereof.
The present invention is also directed to a fuel composition suitable for use in a fuel cell which comprises a liquid hydrocarbon having a boiling range at atmospheric pressure falling between about 77 degrees F. (25 degrees C.) and about 437 degrees F. (225 degrees C.), a total sulfur content of less than 3 ppm, an octane rating of less than 85 (R+M)/2 and containing an effective deposit controlling amount of a nitrogen-containing detergent additive. Preferably the liquid hydrocarbon will predominantly comprise hydrotreated straight run gasoline, rerun alkylate, reformate, hydrotreated FCC gasoline, hydrotreated or desulfurized gasoline, or a mixture containing two or more of these. Additionally, a special fuel cell fuel may be prepared from a mixture of low sulfur gasoline blend streams and hydrotreated FCC light cycle oil, hydrotreated jet, hydrotreated diesel, and/or light coker gas oil. While the hydrocarbons listed may be present as components in gasoline, conventional gasoline is not ideal as a fuel for use in a fuel cell. Conventional gasoline has too high a total sulfur content to serve as a suitable fuel for certain types of fuel cell systems without treatment to remove the sulfur. For example, the catalysts used to convert the hydrogen and oxygen to electricity in a proton exchange membrane fuel cell are very sensitive to even very low levels of sulfur and are rapidly deactivated at the sulfur levels normally present in conventional gasoline which typically falls within the range of from about 50 to 500 ppm sulfur. Even the current reformulated gasoline with about 20 to 30 ppm sulfur would still have too much sulfur. Liquid hydrocarbons most suitable for use as a source of hydrogen for such a fuel cell should have a total sulfur content of less than 3 ppm, preferably less than 1 ppm, and most preferably below 0.5 ppm. While the fuel will contain mostly hydrocarbons, a significant amount of oxygenates, such as alcohols, and other components may also be present. Generally a fuel-soluble, non-volatile carrier can also be present to assist in solubilizing the detergent additive.